In flow cytometry particles such as cells, arranged in a sample stream, pass typically one-by-one through one or more excitation light beams with which the particles interact. Light scattered or emitted by the particles upon interaction with the one or more excitation beams is collected, detected, and analyzed to characterize and differentiate the particles. For example, forward scattering of an excitation beam along its axis may provide information about particle size, side scattering of an excitation beam orthogonally to its axis may provide information about particle internal structure or internal complexity, and fluorescence excited by the one or more excitation beams may provide information about the presence or absence in the particles of fluorophores correlating with particular chemical or biological properties of the particles.
In a sorting flow cytometer particles may be extracted out of the sample stream after having been characterized by their interaction with the one or more excitation beams, and thereby sorted into different groups. Such sorting may utilize, for example, gas jets, electrostatic forces, or other methods to displace selected particles from a sample stream flowing in air.
The performance of flow cytometers may be limited, for example, by the efficiency with which they collect light scattered or emitted by the particles, by susceptibility to misalignment of the their optical components, and by the difficulty with which optical components such as light sources or light detectors, for example, may be replaced or substituted without significantly misaligning or otherwise disrupting the operation of the instrument.